Bauxite ore digestion in the bayer process

ABSTRACT

The process according to the invention to raise simultaneously to an optimum level the temperature and concentration of an aqueous medium intended for use in bauxite ore digestion in an alumina production plant according to the Bayer process, which includes successive phases (a) to (f) in which: 
     a) the aqueous medium coming from digestion containing pregnant liquor is cooled and evaporated in a multiple staged flash evaporation zone; 
     b) the sterile residue is separated, the aluminium trihydroxide is precipitated and the aqueous medium, consisting of spent liquor, intended for use in digestion is recovered; 
     c) if need be, the required quantity of water, additional to that already evaporated in phase (a) is eliminated by evaporation in a specific evaporation section; 
     d) the aqueous medium intended for use in digestion is heated in several stages; 
     e) the flash vapour generated in each of the stages of the evaporation phase (a) is used in each of the stages of the heating phase (d); 
     f) the aqueous medium intended for use in digestion coming from the heating phase (d) is heated by live steam. 
     is characterised in that, in addition to all these phases, a simultaneous heating and evaporation phase is carried out on the aqueous medium intended for use in digestion.

The invention involves the improvement of a continuous bauxite oredigestion process according to the Bayer process which enables thesynchronised adjustment and optimisation of the temperature and theactive caustic soda content in the aqueous medium to be used indigestion (also called attack liquor).

The invention also involves the improvement of the Bayer processdigestion line used to recycle the weak caustic aqueous solution (calledspent liquor) coming from the precipitation of aluminium trihydroxide,heating and concentrating it to an active caustic soda content complyingwith the objectives set by the User for bauxite digestion, in Bayerprocess digesters.

Throughout the description, the following definitions will apply:

the aqueous medium intended for use in digestion means both the spentliquor collected after the precipitation of Al(OH)₃ which is recycledafter heating it and concentrating the active caustic soda to form theattack liquor and the slurry formed by mixing this attack liquor withthe crushed bauxite;

the aqueous medium from digestion means the liquor with a high aluminatecontent coming from bauxite digestion, along with possible unattackedsterile matter to be eliminated, also called pregnant liquor.

STATUS OF THE TECHNIQUE

The Bayer process is the main technique for alumina production via thealkaline attack of bauxite. According to this process, which iscontinuous, the digestion of the bauxite is achieved by bringing aboutthe solubilisation of the alumina under pressure and at an adequatetemperature, in general at least 140° C. (depending on whether digestionis carried out at a low or high temperature, the temperature leveldepends on the grade of bauxite processed—bayerite or hydrargillite orgibbsite, and diaspore or boehmite). This digestion is brought about bymeans of an aqueous caustic soda solution (called attack liquor) whichhas an appropriate concentration of active caustic soda (expressedhereafter in g/l of active Na₂O). After digestion, an aqueous medium isobtained comprising a solution saturated in sodium aluminate (calledpregnant liquor) and sterile impurities to be eliminated. This mediumresulting from the digestion process is gradually cooled by multiplestage flash evaporation, then the pregnant liquor is separated from thephase of impurities which is discarded.

This phase of impurities makes up the unattacked residue of the ore(called red mud), which contains mainly various iron, silica andtitanium oxides and metallic salts.

As for the pregnant liquor, it is cooled down to the requiredtemperature and seeded with aluminium trihydroxide crystals to bringabout the precipitation of the aluminium trihydroxide Al(OH)₃ from thesodium aluminate. After liquid-solid separation, the separated aluminiumtrihydroxide undergoes calcination to produce alumina Al₂O₃ and the weaksodium aluminate liquor (called spent liquor) is sent back to be used indigestion after adequate heating and concentration of its active causticsoda content.

In the Bayer process as it is implemented today, substantial problemsstill exist involving both:

the amount of energy required to bring the attack liquor up to thetemperature level set by the User;

the reconcentration of the active caustic soda content in the spentliquor (to the optimal level for digestion, and appropriate to the settemperature) by the evaporation of a quantity of water ensuring thebalance of the process.

With regard to the quantity of heat required to raise the bauxite attackliquor to the set temperature level, the spent liquor is heated mainlyby recovery of the sensible heat given off during the cooling of thepregnant liquor returning from digestion, in a multiple stage flashevaporation system. But this recovered energy provides the spent liquorwith a certain temperature level which proves insufficient and which hasto be further raised by using live steam in a series of tubular heaters.

To illustrate this phenomenon, in the case for instance of digestion at141° C., the spent liquor, which is heated and concentrated byevaporation in the heating zone (fed with the heat provided by themedium returning from digestion recovered by multiple stage flashevaporation) enters this zone at a temperature of between 80 and 90° C.and leaves it at a temperature of about 120° C.

This spent liquor, still not sufficiently heated, must therefore beraised to about 156° C. (for example) so that the attack medium formedby the mixture of the said liquor and the bauxite (heated to about 90°C.) reach the required temperature of 141° C.: this rise in temperaturefrom 120° C. to 156° C. is achieved by using live steam in tubular heatexchangers.

As for the reconcentration by evaporation of the water from the spentliquor, this is necessary due to the fact that this recycled liquor:

is diluted by intakes of water into the cycle, for example for washingoperations on the impurities and the Al(OH)₃ precipitate

must have an optimal active caustic soda content (expressed in g/l ofactive Na₂O) for ore attack at the set temperature;

must be reconcentrated in such a way that the quantity of water presentin the Bayer bauxite digestion cycle is kept as constant as possible.

This is why the excess water thus generated must be eliminated and thiselimination is carried out by evaporation in at least one multiple stageflash evaporation zone, associated with a zone of tubular heatexchangers heated by flash vapour and, in addition, by live steam.

Thus, to heat and concentrate the spent liquor by evaporation, in thedigester cycle of alumina production units (whether digestion is carriedout at high or low temperatures) there is a multiple stage flashevaporation zone which enables the recovery of heat from the mediumcoming from digestion and its transfer to the medium intended for use indigestion (spent liquor) in a multiple stage flash evaporation zone madeup of tubular heat exchangers in which the last stage is heated by livesteam.

However, in addition to the flash evaporation zone previously mentioned,in many Bayer process alumina production units, there is also a(specific) evaporation section for the spent liquor—multiple stage flash(when the active caustic soda content is low) or multiple effect (inother cases), which completes the action of the said multiple stageflash evaporation zone. This specific evaporation section is fed withlive steam and processes the spent liquor (which is at a temperature ofabout 80 to 90° C.) resulting from the precipiation of Al(OH)₃.

To be precise, the Bayer process, illustrated by the diagrams in FIGS.1, 2 and 3, amounts to a succession of essential steps which, apart froma few slight differences, are to be found in all alumina productionunits operating according to this process.

In the rest of the account of the status of the technique, the heatingand reconcentration of active caustic soda in the spent liquor willconcern indiscriminately the said liquor alone and the slurry formed bythe bauxite to be attacked and the said liquor.

According to FIG. 1 (which does not include a specific evaporationsection), the bauxite intended for use in digestion is ground at point(A) to the adequate size (possibly in presence of a fraction of theattack liquor), then the ground bauxite is mixed at point (B) with theattack liquor forming a slurry which is heated indirectly by live steam.

The slurry thus produced is then gradually heated to the righttemperature for the digestion of the bauxite in the multi-stage heatingzone (C) (comprising tubular heat exchangers) using flash vapour fromthe multiple stage flashing zone (E) and in the final stage live steam.

This slurry is then introduced into the digestion zone (D) where it iskept at the set temperature for attack under pressure: this attack leadsto the production of an aqueous medium consisting of the pregnant liquorand the unattacked impurities.

On leaving the digestion section (D), the slurry containing the pregnantliquor (at a high temperature and pressure) is flashed in the multiplestage flash evaporation zone (E) at gradually decreasing pressures downto a pressure close to atmospheric pressure. The vapour generated byflashing is collected at each pressure level corresponding to one stageand then condensed to be used in the multi-staged heating zone (C) toheat in sequence the attack medium. The slurry resulting from digestion,containing the pregnant liquor, flows counter-current to the attackmedium. Thus, the first flashing stage (E) feeds the correspondingheating stage in zone (C) and so on from stage to stage (see FIG. 3).

But the vapour generated by the first flashing stage, due to itsinsufficient pressure, is unable heat the attack medium to the requiredtemperature. For this reason, the final heating stage of the said attackmedium is carried out by means of live steam just before it enters thedigestion zone (D).

On leaving the multiple stage flash evaporation zone (E), the cooledaqueous medium, containing the pregnant liquor and the unattackedimpurities, enters a dilution zone (F) which collects the waters used towash the unattacked residue (containing sodium aluminate) coming fromzone (N). Then the cooled aqueous medium is fed to a liquid-solidseparation zone (G) where the pregnant liquor is separated from theunattacked impurities which make up the red mud.

The red mud, impregnated with pregnant liquor, is washed in the washingzone (N) and the washing water is sent to the previously mentioneddilution zone (F) to recover the pregnant liquor.

As for the pregnant liquor coming from zone (G), it undergoes a lastseparation in zone (H), it is then cooled in heat exchange zone (I) andfinally seeded in zone (J) by seed crystals taken from separation zone(K). The aluminium trihydroxide Al(OH)₃ (from sodium aluminate)precipitates in the seeded pregnant liquor, placed in the righttemperature conditions. The precipitation medium, comprising spentliquor and the Al(OH)₃ precipitate, undergoes separation in zone (K).The spent liquor is sent to heat exchanger (I) and the aluminiumtrihydroxide, after washing in zone (L), is subjected to calcination inzone (M), to obtain alumina Al₂O₃, the objective of the process.

At the outlet of heat exchanger (I), the spent liquor is fed with theground bauxite into the mixing zone (M) where the active caustic sodacontent is adjusted (to compensate for losses in the digestion cycle).The resulting slurry passes through the different stages of heating,digestion and trihydroxide precipitation as described previously.

According to FIG. 2 which includes, as in FIG. 1, a multiple stage flashevaporation zone (E), but which differs in that it has a specificevaporation section (P), the routing of the bauxite digestion process instages, from the grinding (A) and mixing (B) zones to the aluminiumtrihydroxide calcination zone (M), is the same as that described forFIG. 1.

But, contrary to FIG. 1, on the outlet of heat exchanger (I), the spentliquor is fed to the specific evaporation section (P), which is heatedby live steam. This section is equipped with a multiple stage flashevaporation zone and a multiple stage or multiple effect heating zone,in which the required quantity of water is removed by evaporation, inaddition to that already evaporated in the multiple stage flashevaporation zone (E) to ensure the water balance in the aluminaproduction plant.

In FIG. 3, it is possible to see the staging of the flashing (E) andheating (C) zones and also the vapour links between the flashing zone(E) [from FT₁ to FT_(n)] and the heating zone (C) [from H₁ to H_(n)].

The aqueous medium (pregnant liquor) coming from digestion is cooled byevaporation in the multiple stage flash evaporation zone (FT₁ toFT_(n)), and the liquor with a low sodium aluminate content (spentliquor) collected after the precipitation and separation of Al(OH)₃ isheated in the staged heating zone (H_(n) to H₁) and in heater H₀, fedwith live steam, its temperature is raised to the level required toenable bauxite digestion, before being used in the mixing (B) and/ordigestion (D) zones.

It is thus clearly apparent that:

very large quantities of steam are consumed in the bauxite digestioncycle, and these quantities of steam are not used efficiently norsynchronously, which increases the cost price of the alumina produced.

the User cannot optimise the synchronised adjustment of the temperatureand active caustic concentration of the liquor intended for bauxitedigestion.

It is evident from these major observations that, to reach thetemperature and concentration levels set by the Bayer process for theattack medium, an approximately constant quantity of water (per ton ofalumina produced) must be removed from the bauxite digestion cycle byevaporation. This evaporation phase can either be carried out totally inthe multiple stage flash evaporation zone (E) on the pregnant liquorcoming from the digestion section, or by adding to this multiple stageflash evaporation zone (E) a specific evaporation zone (P) to processthe spent liquor.

In cases where a Bayer process alumina production unit is only equippedwith a multiple stage flash evaporation zone (E), this leaves no room tomanceuvre with regard to increased alumina productivity or to respond tothe physical and chemical variations in the bauxite, as, in either case,to face such demands it would be necessary to increase:

the active caustic soda content of the attack liquor

or the quantity of water to be evaporated, i.e. increase the evaporationcapacity of the multiple stage flash evaporation zone (E). As the totalevaporation capacity of this zone (E) cannot flucuate freely to respondto demand, the installation of a specific evaporation section (P)becomes necessary to increase the total evaporation capacity of theplant: this increased evaporation capacity will raise the live steamconsumption substantially.

In cases where the Bayer process alumina production unit is equippedwith both a multiple stage flash evaporation zone (E) and a specificevaporation section (P), the demand for increased productivity alsomakes it necessary to increase the total evaporation capacity of theplant. To fulfil this requirement, the evaporation capacity of thespecific evaporation section (P) has to be increased, which also bringsabout a substantial increase in the live steam consumption.

To illustrate the status of the technique better, EP-A-0335707 describesa recycling process for the spent liquor which, instead of the series offlash evaporation and associated heating stages, uses a different typeof heat exchanger, implementing a medium (heat transfer fluid), toensure heat transfer between the pregnant liquor and the spent liquor.Even if an increase in the quantity of heat transferred may be achieved,the problem, for example, of the use of the previously mentionedtemperature differential between the last heating stage by flash vapourand the temperature of attack remains set and not completely solved.Consequently, this process does not allow the synchronised control ofthe temperature and of the concentration of active caustic soda of theattack liquor.

We also know from AU-554379 the technique of inserting one or moreevaporation sections in the traditional Bayer process spent liquorrecycling line. The section(s) include(s) at least one tubular heaterfed with live steam associated with an expansion vessel.

This traditional recycling line includes:

several flashing stages intended to cool the pregnant liquor andgenerate vapour;

several heating stages intended to heat the spent liquor which flowscounter-current to the pregnant liquor, each heating stage being fedwith the vapour produced by each flashing stage;

a final heating stage fed with live steam and situated at the outlet ofthe last heating stage fed with flash vapour.

The evaporation section(s) include(s) one or more expansion vesselslocated at the outlet of one or more heating stages.

This line presents major drawbacks such as, for example:

the recycled liquor temperature and concentration levels required forore digestion cannot be reached simultaneously by the means implemented;

only a fraction of the spent liquor is heated to a temperature higherthan that of digestion before being mixed with the aqueous mediumcontaining the bauxite to reach a temperature higher than the digestiontemperature;

the fragmentation of the spent liquor leads to an increase in the numberof tubular heaters required to achieve the desired performance: thismeans a rise in investment costs without providing any advantage as faras energy consumption is concerned.

DESCRIPTION OF THE INVENTION

Consequently, the invention, which concerns the Bayer process, pursuesseveral aims to achieve the improvement of this process and also theimprovement of the corresponding digestion line.

One of the aims of the invention is the simultaneous heating of thespent liquor to the required temperature and its concentration to anoptimum value for the digestion of the bauxite ore.

A further aim of the invention is to make the process flexible to enablequick adaptation to variations in production.

A further aim of the invention is to allow the User to increase theevaporation capacity of the Bayer cycle significantly without asubstantial rise in the live steam consumption.

A further aim of the invention is to provide an additional evaporationcapacity in an existing Bayer plant, at a low investment cost, muchlower than that required to install a specific evaporation section.

Finally, a further aim is to increase the energy efficiency of the Bayerprocess alumina production plant, while at the same time providing alarger quantity of water and protecting the environment by reducingalkaline discharges.

Improvement to the Bayer Process

The process according to the invention to raise simultaneously thetemperature and concentration of an aqueous medium to the optimum levelfor the digestion of a bauxite ore in a Bayer process alumina productionunit, which includes successive phases (a) to (f), in which:

a) the aqueous medium coming from digestion containing the pregnantliquor is cooled and evaporated in a multiple stage flash evaporationzone;

b) the sterile residue is separated, the aluminium trihydroxide isprecipitated and the aqueous medium intended for use in digestion,consisting of spent liquor, is recovered;

c) if need be, a required quantity of water, further to that evaporatedin phase (a), is eliminated by evaporation in a specific evaporationsection;

d) the aqueous medium intended for use in digestion is heated in severalstages;

e) the flash vapour generated in each of the stages of the evaporationphase (a) is used in each of the stages of the heating phase (d);

f) the aqueous medium intended for use in digestion coming from heatingphase (d) is heated by live steam;

is characterised by the fact that, in addition to all these phases, asimultaneous heating and evaporation phase is achieved with the aqueousmedium intended for use in digestion.

It is thus evident that processes for the continuous alkaline attack ofbauxite, according to the status of the technique, including phases:

of multiple stage flash evaporation cooling [phase (a)] of the aqueousmedium coming from digestion,

and of staged heating [phase (d)] of the aqueous medium intended for usein digestion, by use of the vapour produced by successive flashing,

whether they are equipped with a specific evaporation section (as inzone P of FIGS. 2 and 3 which consume live steam), or not,

can be further improved and are improved by the invention thanks to theintegration, into the previously mentioned phases, of a new simultaneousheating and evaporation phase of the aqueous medium intended for use inbauxite digestion, whose results may be measured:

in the obtaining of an aqueous medium intended for use in digestion, forwhich synchronised adjustment may be achieved for the active causticsoda content and temperature, to levels complying with the setobjectives for said digestion;

in the substantial improvement of the energy efficiency of the Bayerprocess by the increase in the total evaporation capacity of the plantwithout any significant rise in the overall steam consumption [thisoverall consumption being defined by the addition of the live steamconsumptions in mixing zone (B), in the specific evaporation zone (P) ifapplicable, and in the staged heating zone of phase (d)].

The simultaneous heating and evaporation phase, according to theinvention, can be inserted at any point:

in the staged heating phase [phase (d) of the process] of the aqueousmedium intended for use in digestion;

and/or in the specific evaporation phase [phase (c) of the process].

The specific evaporation section P [phase (c)] can be equipped with thesame multiple stage flash evaporation apparatus as the evaporation phase(a) and as the staged heating [of phase (d)], or can be comprised of amultiple effect.

If the specific evaporation section is of the staged flashing type, itincludes:

n stages of flash evaporators intended to generate the flash vapour and

n heating stages intended to heat up the spent liquor coming from theprecipitation of Al(OH)₃, each heating stage (which includes at leastone tubular heater) using the vapour generated by each flashing stage ofthe same rank;

at least one final heating stage (which includes at least one tubularheater) fed with live steam and located at the outlet of the firstheating stage fed with flash vapour.

If the specific evaporation section P includes a multiple effect, thesimultaneous heating and evaporation phase is built into the digestionzone and any excess vapour produced by evaporation is used to feed oneof the stages (effects) compatible with its operating pressure. Asignificant reduction in the live steam consumption of this multipleeffect is obtained.

For this reason, according to the invention, this simultaneous heatingand evaporation phase is built into the said staged heating phase of thedigestion cycle and/or into the specific evaporation section, in such away that:

it can receive the aqueous medium (spent liquor) to be heated and/orevaporated from the first stage of the heating zone (fed with livesteam) and/or from at least one of the following stages of the saidheating zone (fed with flash vapour), in particular from the firstheating stage which is fed with flash vapour;

it can feed aqueous medium which has been heated and/or evaporated(attack liquor) to the bauxite digestion zone, or to the first stage(fed with live steam) of the heating zone which, in turn, will feed thebauxite digestion zone;

and/or it can feed aqueous medium (spent liquor) to the heater rank n inthe heating phase (d) when it is built into the specific evaporationsection of phase c).

The best point for the installation of the simultaneous heating andevaporation phase is determined by a prior detailed survey of the plantwhich brings to light the main advantages. However, it has been notedthat the simultaneous heating and evaporation phase according to theinvention is frequently installed between the last heating stage fedwith live steam and the first heating stage fed with flash vapour.

The vapour produced (in additional quantities as compared with thestatus of the technique) by the said simultaneous heating andevaporation phase, can be:

used in situ in said phase according to the invention;

and/or used in at least one of the heating stages of phase c) and/orphase d) and/or in particular, in the first stage of heating fed withlive steam;

and/or used in at least one tubular heater installed for this purposebetween at least two of the tubular heaters rank (K) and (K−1) in phased) and/or phase c) (K having a value of between 1 and n);

and/or consumed, in case of an excess in one of the effects of themultiple effect in phase c) bringing about a reduction in the live steamconsumption of said effect;

and/or used to produce high temperature hot water (from 80 to 98° C. forexample) and/or to heat the spent liquor used to prepare the bauxiteslurry.

Thus, and compared with the status of the technique illustrated in FIGS.1, 2 and 3 the simultaneous heating and evaporation phase according tothe invention is built into one and/or the other of the staged heatingzones c) and/or d) of the Bayer process.

The simultaneous heating and evaporation phase according to theinvention includes at least one stage, that is to say one stage (simpleeffect) or several stages (multiple effects) of simultaneous heating andevaporation, each stage being composed of one or several simultaneousheating and evaporation means operating in series or parallel.

Integration of the Simultaneous Heating and Evaporation Phase in theHeating Zone [Phase (d)] Prior to Digestion

In case, for example, of the presence of one single stage for thesimultaneous heating and evaporation phase according to the invention,the said phase can be preferentially integrated between the first stageof the heating zone (fed with live steam) and the mixing and/or bauxitedigestion zone. The vapour generated by the said phase is used in saidphase.

Finally, it is possible in certain cases to implement one single stagecomplying with the invention, this stage being made up of at least twosimultaneous evaporator-heaters, operating in series or in parallel forthe liquor phase and under the same operating pressure for the vapourphase.

If the simultaneous heating and evaporation phase of the aqueous mediumintended for use in digestion is made up of at least two stages, one ofthese stages can be installed between the outlet of the live steamheating phase [phase (f)] and the inlet of the mixing and/or bauxitedigestion zone and another of the said stages can be located between theoutlet of the first of the stages of the flash vapour heating phase[phase (d)] (vapour generated by the cooling phase a) and the inlet ofthe live steam heating phase [phase (f)].

However, it is also possible that one of the stages being located on theoutlet of the live steam heating phase [phase (f)] and the inlet of themixing and/or bauxite digestion zone, the other stages be placed betweenthe outlet of the first of the stages of the flash vapour heating phase[phase (d)] [vapour generated by the cooling (phase a)] and the inlet ofthe live steam heating phase [phase (f)].

Finally, it is also possible, when the simultaneous heating andevaporation phase includes at least two stages, that the said stages belocated between the inlet of the live steam heating phase [phase (f)]and the outlet of the first of the stages of the flash vapour heatingphase [phase (d)].

When there are at least two stages in the simultaneous heating andevaporation phase according to the invention, the vapour generated byone of the simultaneous heating and evaporation stages is used to feedthe preceding simultaneous heating and evaporation stage., and so onfrom one to the next.

Whatever the number of stages implemented in the simultaneous heatingand evaporation phase for the aqueous medium intended for bauxitedigestion, the first stage of the simultaneous heating and evaporationphase is fed with live steam.

Integration of the Simultaneous Heating and Evaporation Phase into theSpecific Evaporation Section P

When the Bayer process alumina production unit is equipped with aspecific evaporation section in addition to the staged evaporation zoneexisting in the bauxite digestion zone to heat and concentrate the spentliquor, the simultaneous heating and evaporation phase according to theinvention can also be implemented here and can include one or severalstages if the said evaporation zone is equipped with a multiple stageflash evaporation section.

If there is, for example, one single stage (single effect) for thesimultaneous heating and evaporation phase according to the invention,the said phase can be placed between the first stage of the heating zone(fed with live steam) of the said section and the first heating stagefed with flash vapour. The vapour generated by the said phase isconsumed in the said phase.

As already mentioned, it is possible to implement one single stage fromat least two simultaneous evaporator-heaters in the previously givenconditions.

If the simultaneous heating and evaporation phase for the aqueous mediumintended for bauxite digestion includes at least two stages (doubleeffect), one of the stages may be located between the outlet of the livesteam heating phase [phase (c)] and the inlet of the heating stage rankn of heating phase (d) and another of the said stages may be situatedbetween the outlet of the first stage of the flash vapour heating phase(c) and the inlet of the live steam heating stage of phase (c).

But, it is also possible that one of the stages, being placed at theoutlet of the live steam heating phase [phase (c)] and the inlet of theheating stage rank n of heating phase (d), the other stages be locatedbetween the outlet of the first of the flash vapour heating stages [ofphase (c)] and the inlet of the live steam heating phase [of phase (c)].

Finally, if the simultaneous heating and evaporation phase includes atleast two stages, it is possible that the said stages be placed betweenthe inlet of the live steam heating phase (of phase c) and the outlet ofthe first of the stages of flash vapour heating phase [phase (c)].

In case of at least two stages in the simultaneous heating andevaporation phase according to the invention, the vapour generated byone of the simultaneous heating and evaporation stages is used to feedthe preceding simultaneous heating and evaporation phase, and so on fromone to the next.

Whatever the number of stages implemented in the simultaneous heatingand evaporation phase, the first stage of the simultaneous heating andevaporation phase is fed with live steam.

Finally, the excess vapour generated by the stage(s) of the simultaneousheating and evaporation phase can be used as previously indicated, savefor its application in the multiple effect.

The simultaneous heating and evaporation of the aqueous medium intendedfor bauxite digestion in the particular phase according to theinvention, can be carried out preferentially by the use of a tubularfalling film evaporator. This heated film is entrained, from top tobottom, not only by gravity, but also mechanically by the vapourgenerated in situ from the said film resulting in the separation of theconcentrated liquid and the vapour generated in situ.

A previous heating stage of the aqueous medium to be heated andconcentrated may be associated with the simultaneous heating andevaporation phase, upstream of same, by direct contact of the vapourgenerated by the said phase and the aqueous medium.

The process as presented can be used even if the bauxite is introducedinto the spent liquor before it is fed to the multi-stage heating zone.

Improvement of the Bayer Process Digestion Line

The invention also involves a line of apparatus to bring an aqueousmedium intended for Bayer process bauxite ore digestion up to anadequate temperature and concentration by evaporation in an aluminaproduction unit.

As is widely known, this line includes (according to FIG. 3):

a zone of staged evaporators-flash tanks, installed in series, in whichcirculates the aqueous medium coming from bauxite digestion;

a zone of staged heaters, installed in series and in which the aqueousmedium intended for bauxite digestion circulates counter-current to theafore-mentioned zone, each heater being fed with the flash vapour fromeach corresponding evaporator-flash tank;

one or more heaters connected to a source of live steam and linked up tothe inlet of an apparatus for the mixing and/or digestion of groundbauxite with attack liquor, to raise this liquor, coming from the outletof the first of the heaters of this series of heaters fed with flashvapour, to the temperature required for digestion;

a specific evaporation section equipped with a zone comprising amultiple effect or a zone of staged evaporator-flash tanks, in whichflows the aqueous medium (spent liquor coming from Al(OH)₃precipitation) to be heated and evaporated and a zone of staged heatersinstalled in series through which the said spent liquor circulates.,each heater being fed with the flash vapour from the correspondingevaporator-flash tank.

But, according to the invention, this line differs from the status ofthe technique and is characterised by the fact that a simultaneousheating and evaporation section for the aqueous medium intended forbauxite digestion (spent liquor to be heated and concentrated) is builtinto it, both in the heating zone of phase (d) of the Bayer process andin the specific evaporation section of phase (c) of said process, whenthe latter is made up of multiple stage flash evaporators.

This simultaneous heating and evaporation section can be installed atany point in one and/or the other of the phases (c) and (d) of thestaged heating section, but chosen judiciously for each case, after aneeds analysis.

This section according to the invention is connected to the stagedheating section in such a way that:

it can receive the aqueous medium (spent liquor to be heated and/orevaporated) coming from the first stage fed with live steam in the zoneof staged heaters in phases (c) and/or (d) and/or at least one of thefollowing stages of the said heating zone, fed with flash vapour and, inparticular, the first of them;

it can feed the aqueous medium (attack liquor) once it has been heatedand/or evaporated according to the desired and set conditions, eitherdirectly to the zone where mixing and/or bauxite digestion take place,or indirectly via an intermediate connection to the first stage (fedwith live steam) of the heating zone, the latter feeding the aqueousmedium heated and/or evaporated to the digestion zone as required fordigestion, or else to the heater rank n of phase (d) of the process.

The simultaneous heating and evaporation section (for the aqueous mediumintended for use in digestion) in accordance with the invention, is fedpartly with live steam and in turn generates vapour which is:

consumed in the said section;

and/or used in at least one of the heating stages of phases c) and/or d)and/or, in particular, in heater H₀, combining the live steam feed ofthe said heater with all or part of the vapour produced in the saidsimultaneous heating and evaporation section;

and/or used in at least one tubular heater specially installed for thispurpose between at least two heaters in phases (c) and/or (d);

and/or consumed in case of excess production in one of the effects ofthe multiple effect in phase (c);

and/or used to produce hot water to heat the spent liquor entering intothe preparation of bauxite slurry by appropriate means.

The simultaneous heating and evaporation section (of the aqueous mediumintended for use in digestion) includes at least one simultaneousheating and evaporation stage, that is to say a stage (simple effect) orseveral simultaneous heating and evaporation stages (multiple effects),each stage being composed of one or several simultaneous heating andevaporation means operating in series or in parallel. If the sectionaccording to the invention has one single simultaneous heating andevaporation stage (single effect), the said stage can be preferentiallyinstalled:

for phase d) between the first heater (of the heating zone) fed withlive steam and the mixing and/or bauxite digestion zone;

and/or for phase (c) between the first heater fed with live steam andthe inlet of the heater rank n of phase d). The vapour generated by thesaid stage according to the invention is used in situ.

If the simultaneous heating and evaporation section according to theinvention has two stages (double effect) then the two stages can beinstalled

for phase (d) between the outlet of the heater fed with flash vapour atthe highest pressure and the inlet of the mixing and/or bauxitedigestion zone;

and/or for phase (c) between the first heater fed with live steam andthe inlet of the heater rank n of phase (d).

But, it is also possible, in cases where the simultaneous heating andevaporation section includes two stages, that one of the stages beinginstalled at the outlet of the first heater fed with live steam inphases (c) and/or (d), the other stage be installed between the outletof the first of the heaters in the heating zone [phases (c) and/or (d)]fed with flash vapour (generated by the cooling phase) and the inlet ofthe first heater fed with live steam.

Finally, it is also possible, in cases where the simultaneous heatingand evaporation section includes two stages, that these stages beinstalled between the inlet of the first heater fed with live steam andthe outlet of the first of the heaters fed with flash vapour [phase (c)]in the heating zone of phases (c) and/or (d).

If, for example, the section according to the invention includes threestages or more, the first stage is installed as indicated previously, inphases (c) and/or (d). The second and third stages and other stages canbe installed in series between the outlet of the first heater in theseries of heaters fed with flash vapour and the inlet of the heater fedwith live steam.

Whatever the number of stages (effects) in the section according to theinvention:

the first stage of the section is fed with live steam;

the vapour generated in each stage of the section is used to feed thepreceding simultaneous heating and evaporation stage and so on from oneto the next, or yet again to feed vapour, when in excess, to an externaluser as indicated previously.

Preferentially, each simultaneous heating and evaporation stage for theaqueous medium intended for use in digestion is falling film andincludes, in it upper part, a vertical shell and tube heat exchanger anda distribution system for the falling film of aqueous medium intendedfor use in digestion and, in the lower part, a liquid-vapour separatorenabling the separation of the concentrated aqueous medium from thevapour generated, which may be reused.

Each stage of simultaneous heating and evaporation according to theinvention can be preferentially equipped upstream with a heater for thesaid aqueous medium by direct contact of said medium with the vapourgenerated in the said simultaneous heating and evaporation stage. Inthis case, the said direct contact heater is connected to the saidsimultaneous heating and evaporation stage, to ensure the circulation ofthe aqueous medium intended for use in digestion, and the heater bydirect injection of the vapour produced by the said simultaneousheater-evaporator.

Thus a complete plant for alumina production according to the Bayerprocess can be improved thanks to the invention, by the integration ofthe means required for the simultaneous heating and evaporation of theaqueous medium intended for digestion, these means allowing thesynchronised adjustment of the temperature and concentration of theactive caustic soda in the said aqueous medium.

The invention offers the additional advantage of the possibility ofinstalling it in existing alumina production lines, by using simplebranch connections without having to make any other technologicalmodifications to the line.

Other advantages as regards the process and the recycling line accordingto the invention will appear on reading the detailed description of theinvention, while referring to the drawings provided as an illustration,in which:

FIG. 4 shows an alumina production plant according to the status of thetechnique (Bayer process) equipped with the improvement according to theinvention, including:

the successive cooling phases by multiple stage flash evaporation of theaqueous medium coming from digestion, the staged heating of the aqueousmedium going to bauxite digestion, by using the previously mentionedflash vapour;

the simultaneous heating and evaporation phase according to theinvention of the aqueous medium intended for bauxite digestion to raisesimultaneously the temperature and concentration of the active causticsoda in the said medium to the optimum levels.

FIGS. 5 and 6 show the same alumina production plant in accordance withthe status of the technique (Bayer process), equipped with theimprovement according to the invention, in two versions:

one illustrated in FIG. 5, in which the simultaneous heating andevaporation phase according to the invention:

is fed with aqueous medium intended for use in digestion via the outletof the first of the stages of the flash vapour heating phase [phase(d)];

and feeds, after having subjected the said aqueous medium for bauxitedigestion to simultaneous heating and evaporation, to the inlet of thelive steam heating phase (first heater).

the other illustrated in FIG. 6, in which the simultaneous heating andevaporation phase according to the invention:

is fed by the first heater of the heating phase by live steam;

and feeds, after having subjected it to simultaneous heating andevaporation, to the mixing and/or bauxite digestion zone.

FIG. 7 shows a specific evaporation section, installed in an aluminaproduction unit according to the status of the technique (FIG. 3), thesaid specific evaporation section being equipped with the improvementaccording to the invention (phase FFES).

FIG. 8 shows in detail the single stage simultaneous heating andevaporation phase (one effect).

FIG. 9 shows in detail the single stage simultaneous heating andevaporation phase (one effect) including preferentially upstream, adirect contact heating zone for the aqueous medium intended for bauxitedigestion by means of the vapour generated by the said simultaneousheating and evaporation phase.

FIG. 10 shows a version of FIG. 9 including a recirculation (by pump) ofthe aqueous medium intended for use in digestion through thesimultaneous heating and evaporation phase in accordance with example 1.

FIG. 11 shows a version of FIG. 9 including a compressor for the vapourgenerated by the simultaneous heating and evaporation phase.

FIG. 12 shows in detail the double stage simultaneous heating andevaporation phase (double effect) according to the invention, includedin FIG. 4.

FIG. 13 shows a version of FIG. 12 including recirculation of theaqueous medium for bauxite digestion by pump in each stage of thesimultaneous heating and evaporation phase and complying with example 2.

FIG. 14 shows in detail the three stage simultaneous heating andevaporation phase (triple effect).

FIG. 15 shows a version of FIG. 14 including including recirculation ofthe aqueous medium for bauxite digestion by pump in each evaporator ofthe simultaneous heating and evaporation phase and complying withexample 3.

DETAILED DESCRIPTION OF THE INVENTION

As a preliminary statement, it must be remembered that, within the Bayerprocess, there are many industrial design variables in aluminaproduction by alkaline bauxite digestion depending on composition. Thenotable differences between these variations in process mainly concernthe active caustic soda content in the attack liquor, the temperaturelevel of the aqueous medium and the duration of digestion. Theseversions differ also one from the other in the results that they offer,in particular, the production output, the energy efficiency and thequality of the alumina produced.

Since the invention consists in the integration of a simultaneousheating and evaporation section in the digestion line, it applies, withthe same quality of results, to all the versions of the Bayer process,the digestion temperatures and the liquor concentrations as indicated inparticular are not a restriction of the invention.

According to FIGS. 4 and 8, the devices which make up an aluminaproduction plant according to the Bayer process, that is to say, inaccordance with the status of the technique (FIG. 3) they include, as iswidely known, first of all a mixer B, in which the bauxite ore arrivingvia supply line 2 is mixed with at least part of the attack liquor(containing active caustic soda) coming from the recycling line viasupply line 3. The bauxite is previously preheated to a low temperature(between 75 and 80° C.) and the mixture of bauxite and attack liquor isheated in (B) by live steam (fed via lines 5 and 20) from the live steamsource.

In all figures, the circulation routes

for live steam are shown by broken lines

for flash vapour or evaporation vapour are shown by dotted lines

Via line 7 the slurry from (B) is fed to a reactor (D) where the bauxiteis digested by the hot attack liquor (at about 145° C.) under pressure.To ensure the optimum concentration of caustic soda in the attack liquor(compensation for losses occurring throughout the alumina productionprocess), mixing zone B if fed not only with attack liquor (via line 3)but also with fresh active caustic soda.

From reactor D the aqueous medium coming from digestion, made up ofpregnant liquor containing sodium aluminate in solution and unattackedsterile residue, is removed via line 8 and sent to a series (E) of flashtanks-evaporators FT, operating according to the staged flashingprinciple. The successive flash tanks-evaporators FT₁ to FT_(n)(connected in series) produce vapour, by flashing the aqueous mediumfrom digestion, which is used to heat a series (C) of tubular stagedheaters H₁ to H_(n) (also connected in series).

Thus, flash tank-evaporator FT₁ receives the aqueous medium resultingfrom digestion at a temperature of about 145° C. via line 8. In flashtank-evaporator FT₁, the temperature of said aqueous medium falls fromabout 145 to 135° C. and produces flash vapour. This vapour, at atemperature of about 128° C., is fed to heater H₁ via line 9.

Similarly, flash tank-evaporator FT₂ receives the aqueous medium comingfrom FT₁ at a temperature of about 135° C. via line 11. In flashtank-evaporator FT₂ vapour is produced by flashing the aqueous mediumand is fed via line 12 to heater H₂. The same occurs in flashtank-evaporator FT₃, which receives the hot aqueous medium from flashtank-evaporator FT₂ via line 13 and feeds flash vapour to heater H₃ vialine 14.

The flashing cycle continues thus: the flash tank-evaporator FT_(n)receives the aqueous medium from flash tank-evaporator FT_(n−1) via line16 and feeds flash vapour via line 17 to heater H_(n).

The flash cooled aqueous medium is then sent via line 10 to asolid-liquid separation section F in which the pregnant liquor isseparated from the red mud. The said pregnant liquor, at a temperatureof about 75-80° C. and void of insolubles, then passes through line 15to section K for aluminium trihydroxide Al(OH)₃ precipitation(subsequently transformed into alumina Al₂O₃ by calcination).

The precipitation of the Al(OH)₃ in section K is followed byliquid-solid separation which produces spent liquor, in addition to thealuminium trihydroxide. The active caustic soda content of this spentliquor, intended for recycling to bauxite digestion, must bereconcentrated by water evaporation and raised to a temperature higherthan that required for bauxite digestion so as not to disturb thephysical and chemical conditions of the attack medium. Thus the spentliquor is sent from the precipitation-separation section K to a firstheater H_(n) via line 18.

The spent liquor then circulates from heater H_(n) to heater H_(n−1) vialine 19, and so on in series, from heater H₃ to heater H₂ via line 21,then from heater H₂ to heater H₁ via line 22. Each heater H is fed withvapour from the flash tank-evaporator FT which is directly associatedwith it. On the outlet of heater H₁ the temperature of the liquorintended for use in digestion is about 120° C. Thus, the spent liquor tobe regenerated passes from heating stage to heating stage from about 80°C. to about 120° C., but its temperature is still too low for it to beused as attack liquor.

As the temperature of the bauxite fed to B is insufficient, thetemperature of the attack liquor must be raised from about 120° C. toabout 156° C. to obtain a temperature of about 145° C. in the attackmedium, formed by the mixture of bauxite and the attack liquor, at theactual time of digestion. To achieve this, at least one tubular heaterH₀ is used connected via line 23 (see FIG. 3) to heater H₁ and to thelive steam source via line 24. The liquor intended for use in digestionis fed from heater H₀ via line 3 to mixing zone B, which in turn feedsthe digester (D) via line 7. The digestion cycle is thus completed andthe alumina production unit is in continuous operation.

According to the invention (FIG. 4) a new simultaneous heating andevaporation section (Falling-Film Evaporation Section) for aqueousattack medium is built into the previously mentioned Bayer processdigestion unit. Thus a branch connection is made between heaters H₁ andH₀ and between heater H₀ and the mixing zone (B). From heater H₁ thespent liquor is fed via line 26 to the FFES section (and no longerpasses through line 23 as per the status of the technique according toFIG. 3. The aqueous medium for digestion is fed via line 27 from thesimultaneous heating and evaporation section FFES to heater H₀ (fed withlive steam via line 24). At the outlet of heater H₀ line 28 feeds theaqueous medium for digestion to the simultaneous heating and evaporationsection FFES. The aqueous medium for digestion leaves the FFES sectionvia line 29 at a temperature and concentration appropriate for digestionand enters the mixing zone B from where it is fed via line 7 to thedigester (D).

The simultaneous heating and evaporation section FFES is fed with livesteam via line 36. The vapour generated in the simultaneous heating andevaporation section FFES by the evaporation of the aqueous mediumintended for use in digestion is consumed in situ and any excess isrecovered via line 37 for at least one of the uses mentioned previously.

According to FIG. 5, specific to the invention, the simultaneous heatingand evaporation section is also built into the Bayer process digestionsection. Thus a branch connection was made between heaters H₁ and H₀.From heater H₁ the spent liquor is fed via line 26 to the FFES section.Via line 27 the aqueous medium for digestion is fed from thesimultaneous heating and evaporation section FFES to heater H₀ (fed withlive steam by line 24). At the outlet of H₀ (differing from FIG. 4) line29 takes the aqueous medium for digestion at optimum temperature andconcentration, partly to the mixing zone B and partly to the digester D,while the slurry leaving B is fed via line 7.

According to FIG. 6, concerning the invention, the simultaneous heatingand evaporation section is also built into the Bayer process digestionsection. But, unlike FIGS. 4 and 5, a branch connection is made betweenheater H₀ and mixing zone B. From heater H₁ the spent liquor flowsthrough line 23 to heater H₀ (which is fed with live steam by line 24).On the outlet of heater H₀, a line 28 brings the aqueous medium forattach to the simultaneous heating and evaporation section FFES. Line 29leaving section FFES takes the aqueous medium for digestion, now atoptimum temperature and concentration for digestion, to mixing zone Bthen to digester D.

FIG. 7, applying to the invention, shows the case where the simultaneousheating and evaporation section FFES is built into a specificevaporation section (P), existing in an alumina production unitaccording to FIG. 3 of the status of the technique.

According to this FIG. 7, which concerns phase c) of the Bayer process,the specific evaporation section (P) is equipped with a series (104) ofstaged heaters with increasing temperatures (H_(n) to H₁), and a series(101) of multiple stage flash evaporators (FT₁ to FT_(n)).

The spent liquor, coming from the Al(OH)₃ precipitation and separationzone (I). enters via line 118 into heater H_(n) of the said section, andprogressively heats as it flows from heater H_(n) to heater H₁ via lines119, 121 and 122.

The simultaneous heating and evaporation section according to theinvention is integrated into this set up by means of a branch connectionbetween heaters H₁ and H₀ on one hand and between heater H₀ and flashevaporator FT₁ on the other hand.

From heater H₁, the spent liquor flows via line 126 to the FFES section(and no longer via line 23 of the status of the technique as per FIG.3). The spent liquor is sent via line 127 from the simultaneous heatingand evaporation section FFES to heater H₀ (fed with live steam by line124). On the outlet of heater H₀ line 128 takes the aqueous mediumintended for use in digestion to the simultaneous heating andevaporation section FFES. On leaving the FFES section via line 129 theliquor has reached the optimum temperature and concentration in theseries of staged flash evaporators.

The successive staged flash evaporators FT₁ to FT_(n) (connected inseries) produce vapour by flashing the liquor coming from the FFESsection which is used to heat the series 104 of staged tubular heatersH₁ to H_(n). Thus the flashing cycle is achieved from one to the next,the liquor flowing from evaporator FT₁ to evaporator FT_(n) via lines111, 113, 116 . . .

The liquor cooled by staged flashing, leaving FT_(n) is reintroduced vialine 110 upstream of heater H_(n) of phase d) of the Bayer process. Eachheater H is fed with vapour from the flash evaporator FT which isdirectly associated with it, heater H₀ being fed with live steam.

The simultaneous heating and evaporation section FFES is fed with livesteam by line 136 from the source of live steam. The vapour generated inthe simultaneous heating and evaporation section FFES by the evaporationof the liquor to be heated and concentrated, is used in situ in the saidphase.

In case of a possible excess of vapour generated in the FFES section,this excess is sent by line 137 to at least one of the vapour consumingzones, such as those indicated previously, for example phase (d) of theBayer process.

FIGS. 8 to 15 of the simultaneous heating and evaporation section withone or more stages, described hereafter, concern the integration of thesaid section both in phase (c) (specific evaporation section) and phase(d) (digestion section) of the Bayer process: the description wasdeliberately restricted to phase (d) of the said process, but it alsoapplies in the same way to phase (c) of the said process when the latteris equipped with multiple stage flash evaporation.

FIG. 8, specific to the invention, illustrates the case in which thesimultaneous heating and evaporation section FFES has one single stage(39) (that is to say a single effect). This stage 39 is inserted betweenheater H₀ and mixing zone B and includes a simultaneousheater-evaporator 54 which operates according to the falling filmprinciple.

The spent liquor coming from heater H₀ is fed to the top of the saidsimultaneous heater-evaporator 54 via line 28.

Simultaneous heater-evaporator 54 includes a vertical shell and tubesurface heat exchanger 56 and a liquid-vapour separator 53. Exchanger 56is equipped with numerous tubes with a nominal bore of between 30-50 mmand a length of between 8 to 15 metres. The spent liquor is distributedin the upper section by means of an appropriate system, and the mixtureformed by the said liquor and the vapour falls down the length of thetubes. The liquor concentrates by evaporation and leaves via the bottomof the tubes from where it flows to the liquid-vapour separator 53. Thevapour coming from liquid-vapour separation in 53 is partly used to feedtubular heater H₀ via line 57, the other part is reinjected via line 59into the upper section of the simultaneous heater-evaporator. Theheating zone of exchanger 56 is fed with steam from the live steamproduction unit LS.

The bottom of the liquid-vapour separator 53 is equipped with a pipe 29which sends the liquor, at the optimum temperature and concentration fordigestion, to the mixing (B) and digestion (D) zones.

FIG. 9, specific to the invention, shows another version of FIG. 8.According to this figure, the simultaneous heating and evaporationsection FFES includes one single stage 39 (single effect). This stage isinserted between heater H₀ and mixing zone (B). The simultaneous heatingand evaporation stage 39 includes a simultaneous heater-evaporator whichoperates according to the falling film principle and an associatedheater-mixer 52 operates according to the direct contact principle. Thistype of simultaneous heater-evaporator and its associated direct contactheater is described in patent FR-A-361524 and is taken up again inpatent FR-A-1419663 (or also in the publication: “LIGHT METAL 1994,Proceedings of the technical sessions presented by the T.M.S. LightMetal Committee at the 123 Rd-TMS Annual Meeting, San Francisco”,entitled: “Evaporation techniques in the alumina industry”).

Thus simultaneous heating and evaporation stage 39 includes a directcontact heater 52 and a simultaneous heater-evaporator 54. The spentliquor coming from heater H₀ enters the upper part of direct contactheater 52 via line 28. Vapour generated by the simultaneousheater-evaporator 54 (by evaporation of the spent liquor) is thus, atleast partly, injected directly into the direct contact heater 52 vialine 58, while the other part of this vapour goes via line 57 to feedheater H₀. The spent liquor which is heated by the direct contact heater52 to its boiling point is then fed via line 55 to simultaneousheater-evaporator 54.

For the remainder of the description, FIG. 9 complies with FIG. 6.

FIG. 10, specific to the invention, is also another version of FIGS. 6and 8. According to this figure, the simultaneous heating andevaporation section FFES includes a stage 39. This stage is insertedbetween heaters H₀ and mixing zone (B). The simultaneous heating andevaporation stage 39 used includes a simultaneous heater-evaporatorwhich operates according to the falling film principle andpreferentially a heater-mixer (52) is associated with it upstream,operating according to the liquid-vapour direct contact principle.

The spent liquor coming from heater H₀ enters the upper part of thedirect contact heater 52 via line 28. Vapour generated by simultaneousheater-evaporator 54 (by evaporation of the attack liquor) is thus, atleast partly, injected directly into the direct contact heater via line58, while the other part of this vapour goes via line 57 to feed heaterH₀. The temperature of the spent liquor is thus raised to boiling pointby the direct contact heater 52 and then flows via line 55 intosimultaneous heater-evaporator 54.

The simultaneous heater-evaporator 54 includes a vertical tubularsurface heat exchanger 56 and a liquid-vapour separator 53. Unlike FIGS.8 and 9, the decomposed liquor is recycled by means of pump 60 (whichensure a constant flow) from separator 53, it flows through line 61 andenters vertical tubular exchanger 56 via a distribution system 62.Exchanger 56 has long vertical tubes as previously indicated. Themixture of spent liquor and vapour falls down the length of the tubes,concentrates by evaporation and leaves the bottom of the tubes thenreturns to the liquid-vapour separator 53. The vapour coming fromliquid-vapour separation is used to feed the direct contact heater 52via line 58 and tubular heater H₀ via line 57. The heating zone ofexchanger 54 is fed with live steam coming from the live steamproduction unit LS via line 36.

The bottom of the liquid-vapour separator 53 is equipped with aconnection 29 which sends the attack liquor brought up to its optimumtemperature and concentration to the mixing zone B.

FIG. 11, specific to the invention, is also another version of FIGS. 8,9 and 10 in which the simultaneous heating and evaporation section FFESis equipped with a compression station 65 for the vapour generated bythe simultaneous heating and evaporation stage 39, which is used tocompress the vapour coming from separator 53 to the operating pressureof exchanger 54. This compression station is fed by line 66 which takesup part of the vapour coming from liquid-vapour separator 53. On theoutlet of the compressor, the vapour compressed to the required pressureis sent via line 67 and used to heat exchanger 56.

According to FIGS. 12 and 13 which illustrate the invention, thesimultaneous heating and evaporation section FFES includes two stages 39and 49 (double effect). One of the stages 49 is inserted between heatersH₁ and H₀. The other stage 39 is placed between heater H₀ and the mixingzone B. The simultaneous heating and evaporation stages 39 and 49 usedeach include a simultaneous heater-evaporator which operates accordingto the falling film principle and an associated heater-mixer operatingaccording to the liquid-vapour direct contact principle.

The simultaneous heating and evaporation stage 49 includes a directcontact heater 41 and a simultaneous heater-evaporator 44. The spentliquor coming from heater H₁ enters the upper part of direct contactheater 41 via line 26. Vapour generated by simultaneousheater-evaporator 44 (by evaporation of the spent liquor) is injecteddirectly into the direct contact heater 41 via line 42. The temperatureof the spent liquor is thus raised to its boiling point by the directcontact heater 41 and thereafter flows via line 43 into the simultaneousheater-evaporator 44.

The simultaneous heater-evaporator 44 includes a vertical tubularsurface heat exchanger 46 and a liquid-vapour separator 47. Theexchanger 46 is equipped with a large number of vertical tubes with thepreviously indicated characteristics. The mixture of spent liquor andvapour falls down the length of the tube walls, its concentrates andleaves the bottom of the tubes via the liquid-vapour separator 47. Thevapour coming from liquid-vapour separation goes via line 42 to feed thedirect contact heater 41. The heating zone of the exchanger 46 is fedwith vapour from the simultaneous heating and evaporation unit 39 vialine 57.

The bottom of the liquid-vapour separator 47 is equipped with connection27 which sends the spent liquor during regeneration to tubular heaterH₀. The liquor leaving heater H₀ then enters the other simultaneousheating and evaporation stage 39 via line 28. Similarly to stage 49,this stage 39 includes a direct contact heater 52 coupled up to asimultaneous heater-evaporator 54. The simultaneous heater-evaporator54, fed with live steam via line 36, includes a liquid-vapour separator53 and a vertical tube falling film exchanger 56.

Similarly, the vapour produced by evaporation in the simultaneousheater-evaporator 54 is separated and, partly, injected directly intodirect contact heater 52 via line 58. The other part of this vapourflows via line 57 and is used to heat the vertical tubular exchanger 46upstream in the simultaneous heating and evaporation unit 49 and tubularexchanger H₀.

The liquor, heated and concentrated to optimum levels for digestion,makes up the attack liquor which leaves the simultaneous heating andevaporation section FFES via line 29. After having undergone therecycling process according to the invention, this liquor ready fordigestion can return to the mixing stage B then to digestion D of thebauxite and thus start a new cycle.

In FIG. 13, the spent liquor is recycled by means of pump 48 whichensures a constant flow from separator 47, it goes via line 63 to theupper part of exchanger 46 equipped with a distribution system 51.

In FIGS. 14 and 15 which illustrate the invention, a new three stage(triple effect) simultaneous heating and evaporation section FFES forthe aqueous medium used in digestion is inserted in the Bayer processdigestion unit.

In accordance with the indications given previously for the casesdescribed for FIGS. 12 and 13, a simultaneous heating and evaporationstage 39 fed with live steam is inserted between heater H₀ and themixing zone B. The other two stages 49 and 89, installed in line, areinserted between heater H₁ (fed with flash vapour from FT₁,) and heaterH₀ (fed with flash vapour from stage 39).

Each simultaneous heating and evaporation stage (39, 49, 89) includes asimultaneous heater-evaporator which operates according to the fallingfilm principle and an associated heater-mixer (52, 41, 72) operatingaccording to the liquid-vapour direct contact principle (all the meanshave been described for previous cases).

The first stage 39 of the simultaneous heating and evaporation stageaccording to the invention is fed with live steam via line 36. Thevapour it produces itself is sent, partly, to heater-mixer 52 and theother part to heater-evaporator 44 (of the second stage 49) via line 57and to the tubular heater H₀.

The second stage 49 of the said FFES section according to the invention,fed with vapour from the first stage, generates vapour itself which issent, partly, to its heater-mixer 41 via line 42 and the other part issent to heater-evaporator 78 (of the third stage 89) via line 37.

The third stage 89 of the said FFES section according to the invention,fed with vapour by the second stage, also generates vapour which issent, partly, to its heater-mixer 72 via line 87 and, the other part issent via line 84 to at least one of the zones where vapour is consumed,as indicated previously.

The spent liquor leaving H₁ is fed by line 81 to heater-mixer 72 inwhich it is heated by vapour coming from line 87. Thus heated, thisliquor leaving heater-mixer 72 via line 88, enters simultaneousheater-evaporator 78 in which it circulates (FIG. 15) via pump 75 andline 77 and where it concentrates by evaporation-separation (79, 80, 73)and leaves via line 74 and thereafter successively feeds stages 2 and 1in order, as described for the previous cases.

At the end of the cycle, the liquor, which has been heated andconcentrated in the three stages of the simultaneous heating andevaporation section according to the invention, makes up the bauxiteattack liquor, having the temperature and concentration parametersinitially set by the User of the Bayer process.

Generally speaking, the installation of the simultaneous heating andevaporation section according to the invention in the Bayer processalumina production cycle presents the significant advantage, thanks tothe presence of valves such as 75, 31, 32, 33, 34 which are located atthe interconnection of lines 23 and 74, 23 and 26, 23 and 27, 3 and 28,3 and 29 respectively, of enabling the disconnection of one or other ofthe stages of the simultaneous heating and evaporation section FFES ofthe recycling line C and to revert to the usual heating mode, forexample during a cleaning cycle.

Consequently, there is no necessity to invest in a stand-by simultaneousheater-evaporator as is usually the case for tubular heaters.

OPERATION OF THE UNIT ACCORDING TO THE INVENTION

The process according to the invention allows all the disadvantages ofthe previous art to be completely avoided by modifying the system ofexchange between the evaporation vapour and the aqueous medium intendedfor use in digestion (spent liquor transformed into attack liquor),which must be heated and reconcentrated in active caustic soda. Thisaqueous medium, according to FIG. 13 for example, is fed via line 26 todirect contact heater 41, where it is heated by part of the evaporationvapour taken from liquid-vapour separator 47. The aqueous medium is nolonger heated indirectly via the walls of a heat exchanger, but bydirect injection of vapour from the simultaneous heater-evaporator 44into direct contact heater 41 without any heat exchange surfaceintervening. The aqueous medium to be evaporated is heated to itsboiling point before it is fed to the tubular exchanger 46. The effectof this heating is significant and brings about the increased thermalefficiency of the heat exchange surface of the evaporator-heaters.

The aqueous medium (to be used for digestion), thus heated, can then befed through vessel 47, then, by recycling, through the tubular exchanger46 of a simultaneous heater-evaporator. As this operation is carried outcontinuously, the level in vessel 47 is kept constant by thesimultaneous routing of aqueous medium to the first stage ofsimultaneous heating and evaporation and to the tubular exchanger 46 ofsimultaneous heater-evaporator 44, regardless of the load of the latter.

In theory, any type of simultaneous heater-evaporator can be used but inpractice a falling film type simultaneous heater-evaporator ispreferably used. In the latter type of heater-evaporator, the water inthe aqueous medium vaporises in the exchanger tubes as soon as themedium enters the said tubes. The vapour generated entrains the aqueousmedium at increasing velocities forming a film on the inside wall of thetubes of the exchanger. Distribution system 51 operates in such a waythat a practically constant flow of fluid is fed to each tube. Thedesign of the simultaneous heater-evaporator is such that the film issufficiently thin to obtain a good heat exchange coefficient andsufficiently thick to avoid the falling film drying up. The heated andconcentrated recycled aqueous medium flows to vessel 47 which plays therole of liquid-vapour separator.

The vapour thus produced is put into contact with the spent liquorcoming from the previous stage H₁ in direct contact heater 41. Thesimultaneous heater-evaporator operates with evaporation vapour if it isan intermediate stage, or with live steam coming from a generator, if itis a first stage. The particularity of the falling filmheater-evaporator is that the heat required to heat and/or evaporate theliquor is transferred by one single apparatus. Thus the concentration ofthe aqueous medium is achieved in the stage under consideration. Thevapour produced is sent to the evaporator of the previous stage to allowthe heater-evaporator of this stage to play its role fully. Thisconsequently provides a multiple effect heater-evaporator in which theconcentration of the recycled aqueous medium increases from stage tostage.

Thanks to this principle, a significantly greater additional evaporationmay be achieved as compared with existing capacities, especially if itis taken into account that the evaporation capacity of digestion isfixed. Furthermore, the heat transfer coefficient of a simultaneousheater-evaporator, in particular falling film type, is very high (abouttwice that of tubular heaters). This particularity further heightens theeconomic significance of the invention.

The process and the recycling line according to the invention are notrestricted by details of modes of implementation or by the exampleschosen to illustrate them. Modifications may be made without goingbeyond the scope of the invention. The latter consequently includes allthe means which make up technical equivalents of the means described, aswell as their combination.

COMPARATIVE EXAMPLES OF IMPLEMENTATION Comparative Example No. 1Complying with FIGS. 6 and 8

According to the status of the technique, a conventional aluminaproduction plant not equipped with a specific evaporation section (P)according to FIG. 3, with a capacity of 500,000 t/year according to theprevious art (table 1, column 1) includes:

a flash-evaporation section in stages FT, interconnected, installed inseries in which circulates the aqueous medium coming from digestion;

a section of heaters (H), interconnected, installed in series and inwhich circulates the spent liquor (aqueous medium intended for use indigestion) counter-current to the aqueous medium coming from digestion,each flash tank (FT) being connected for vapour circulation with adirectly corresponding heater (H);

a heater H₀ connected to a source of live steam (LS), connected to theinlet of mixer (B) [to ensure the circulation of the aqueous mediumintended for use in digestion coming from the outlet of heater H₁ of theseries of heaters (H)]

The spent liquor (coming from the precipitation of Al(OH)₃) has:

a flowrate of about 1460 t/hr at the outlet of H,

a concentration of about 130 g/l active Na₂O at the inlet of H₁,

a temperature of about 120° C. at the outlet of H₁,

a temperature of about 156° C. at the outlet of H₀,

finally, a concentration of about 130 g/l active Na₂O at the outlet ofH₀.

To raise the temperature of the spent liquor from 120° C. (at the outletof H₁) to 156° C. (at the outlet of H₀), so that the attack liquor (madeup of the mixture of the said liquor with the heated bauxite) reachesthe temperature level required for bauxite digestion (about 145° C.),the evaporation capacity of the staged flashing zone is 86.3 t/hr and itconsumes 91.9 t/hr of live steam for the operation of the plant.

In the same conventional alumina production unit, but equipped with aspecific evaporation section (P) (according to FIGS. 2 and 3), having acapacity of 500,000 t/year complying with the previous art (table 1,column 2), and the previously mentioned conditions, the temperature riseof the spent liquor from 120° C. (at the outlet of H₁) to 156° C. (atthe outlet of H₀), for the attack medium, made up of the mixture of thesaid liquor and the heated bauxite, to be at the required temperaturefor the digestion of this bauxite (about 145° C.), the total evaporationcapacity of the plant is still 86.3 t/hr and it consumes 91.9 t/hr oflive steam for the operation of the said plant.

The Bayer process industrial plant is therefore set to operate at aconcentration of 136 g/l active Na₂O in the spent liquor instead of 130g/l, at the outlet of the heating section (table 1, column 3, line 6).

According to the status of the technique, to increase the active Na₂Oconcentration in the spent liquor from 130 g/l to 136 g/l (table 1,column 2) in this same plant according to the previous art, whilemaintaining the temperature of the liquor at the outlet of H₀ at 156°C., the total evaporation capacity of the plant must be increased from86.3 t/hr to 141.9 t/hr (line 9), which would require an increase of55.6 t/hr in evaporation capacity in the plant (column 3, line 10).

To achieve the set objective, and accelerate bauxite digestion [in theplant according to the status of the technique, equipped with a specificevaporation section (P)] (table 1, column 4), the total evaporationcapacity required in the plant must be increased from 86.3 t/hr to 141.9t/hr. The evaporation capacity of the specific evaporation section (P)must therefore be raised by 55.6 t/hr. Consequently, the total livesteam consumption in the plant increases by 17 t/hr resulting in aconsumption of 108.9 t/hr instead of 91.9 t/hr.

Thus, in a Bayer process plant set up according to the status of thetechnique, an increase of 6 g/l in the active Na₂O concentration of theattack liquor, brings about an increase of 7 t/hr in the live steamconsumption, which renders the said increase in concentration of thespent liquor (or liquor intended for use in digestion) economicalundesirable.

According to the invention, the previously mentioned industrial unit wasequipped with the single stage spent liquor simultaneous heating andevaporation section (FFES). The comparison of figures between theprevious art (table 1, column 4) and the subject of this invention(table 1, column 5) shows that, by implementing the simultaneousheater-evaporator (FFES), the spent liquor temperature being raised from120 to 156° C. (H₀ outlet), it was possible to reach the set objective,that is to say, an increase in the concentration of active Na₂O from 130to 140 g/l in the spent liquor. To achieve this, the total evaporationcapacity of the flashing zone (E) (line 7) was raised from 86.6 t/hr to141.9 t/hr, which represents an increase in total evaporation capacityof 55.6 t/hr as compared with the previous art (columns 1 and 2) for anincreased steam consumption of 0.5 t/hr (92.5 t/hr against 91.9 t/hr)(line 13, columns 1, 2, 4 and 5).

Thus, by means of the invention, with an additional live steamconsumption of 0.5 t/hr as compared with the status of the technique,the evaporation capacity of the plant is increased by 55.6 t/hr.Consequently, the set objective of increasing the active Na₂Oconcentration in the spent liquor intended for attack is reached with avery slight rise in the live steam consumption (0.5 t/hr), whereas thesame plant, not equipped with the invention, could only reach thisobjective with an additional live steam consumption of 17 t/hr.

The integration in the heating section of an alumina production unit(according to the previous art) of a simultaneous heating andevaporation section according to the invention. allows the simultaneousadjustment of active Na₂O in the spent liquor intended for use indigestion, and the liquor temperature to the right level for aluminaproduction and, consequently, considerably improves the overall energyefficiency of the Bayer process with a low investment cost, which can bequickly paid off.

TABLE 1 BAYER PROCESS Plant operating according to previous art presentstatus (1) present status (2) Solution to achieve set objective withoutspecific with specific Objective set for according to according to theevaporation evaporation process operation previous art invention with 1Line section P section P (3) (4) stage (5)  1 Annual production of Al₂O₃(t/hr) 500000 500000 500000 500000 500000  2 Spent liquor flow - outletH₁ (t/hr) 1460 1460 1460 1460 1460  3 Concentration of active Na₂O (ing/l) in spent 130 130 130 130 130 liquor - inlet H₁  4 Liquortemperature - outlet H₁ (° C.) 120 120 120 120 120  5 Liquortemperature - oulet heating section (H₁ or 156 156 156 156 156 FFES) (°C.)  6 Na₂O concentration in spent liquor (in g/l) - outlet 130 130 136136 136 of heating section  7 Evaporation capacity (in t/hr) of thestaged 86.3 86.3 — 86.3 141.9 flashing zone (E)(*)  8 Evaporationcapacity (in t/hr) of the evaporation 0 0(**) — 55.6 (in P) 0 zone (P)heated with live steam  9 Total evaporation capacity of the plant (E) +(P) 86.3 86.3 141.9 141.9 141.9 (in t/hr) 10 Increase in totalevaporation capacity of the plant 0 0 55.6 — 55.6 (in t/hr) 11 Livesteam consumption in zone (E) (in t/hr) 91.9 91.9 — 91.9 92.4 12 Livesteam consumption in zone (P) (in t/hr) 0 0 — 17 0 13 Total live steamconsumption in (E) + (P) (in t/hr) 91.9 91.9 — 108.9 92.4 (*)Theevaporation capacity of zone E is calculated to account for the flashingof the pregnant liquor from 145° C. to 107.3° C. (atmospheric pressure)(**)The hypothesis is made to simplify the explanation that theevaporation section (P) has a capacity of zero before setting theobjective of increasing the active Na₂O concentration in the spentliquor by 6 g/l

Comparative Example No. 2 Complying with FIGS. 4 and 13

According to the status of the technique, a conventional aluminaproduction plant not equipped with a specific evaporation section (P)according to FIGS. 1 and 3, with a capacity of 500,000 t/year accordingto the previous art (table 2, column 1) includes:

a flash-evaporation section in stages FT, interconnected, installed inseries in which circulates the aqueous medium coming from digestion;

a section of heaters (H), interconnected, installed in series and inwhich circulates the spent liquor (aqueous medium intended for use indigestion) counter-current to the aqueous medium coming from digestion,each flash tank (FT) being connected for vapour circulation with adirectly corresponding heater (H);

a heater H₀ connected to a source of live steam (LS), and also to theinlet of mixer (M) [to ensure the circulation of the aqueous mediumintended for use in digestion coming from the outlet of heater H₁ of theseries of heaters (H)];

The spent liquor (coming from the precipitation of Al(OH)₃) has:

a flowrate of about 1460 t/hr at the outlet of H,

a concentration of about 130 g/l active Na₂O at the inlet of H₁,

a temperature of about 120° C. at the outlet of H₁,

a temperature of about 156° C. at the outlet of H₀,

finally, a concentration of about 130 g/l active Na₂O at the outlet ofH₀

To raise the temperature of the spent liquor from 120° C. (at the outletof H₁) to 156° C. (at the outlet of H₀), so that the attack liquor (madeup of the mixture of the said liquor with the heated bauxite) reachesthe temperature level required for bauxite digestion (about 145° C.),the evaporation capacity of the staged flashing zone is 86.3 t/hr and itconsumes 91.9 t/hr of live steam for the operation of the plant.

In the same conventional alumina production unit, but equipped with aspecific evaporation section (P) (according to FIGS. 2 and 3), having acapacity of 500,000 t/year complying with the previous art (table 2,column 2), and the previously mentioned conditions, the temperature riseof the spent liquor from 120° C. (at the outlet of H₁) to 156° C. (atthe outlet of H₀), for the attack medium, made up of the mixture of thesaid liquor and the heated bauxite, to be at the required temperaturefor the digestion of this bauxite (about 145° C.), the total evaporationcapacity of the plant is still 86.3 t/hr and it consumes 91.9 t/hr oflive steam for the operation of the said plant.

The Bayer process industrial plant is therefore set to operate at aconcentration of 140 g/l active Na₂O in the spent liquor instead of 130g/l, at the outlet of the heating section (table 2, column 3, line 6).

In this same plant according to the previous art, to increase the activeNa₂O concentration in the spent liquor from 130 g/l to 140 g/l (table 2,column 2) while maintaining the temperature of the liquor at the outletof H₀ at 156° C., the total evaporation capacity of the plant must beincreased from 86.3 t/hr to 170.3 t/hr (line 9), which requires anincrease of 84 t/hr total evaporation capacity in the plant (column 3,line 10).

To achieve the set objective, that is to say to increase theconcentration of active Na₂O in the spent liquor from 130 g/l to 140 g/lto accelerate bauxite digestion [in the plant according to the status ofthe technique, equipped with a specific evaporation section (P)] (table2, column 4), the total evaporation capacity required in the plant mustbe increased from 86.3 t/hr to 170.3 t/hr. The evaporation capacity ofthe specific evaporation section (P) must therefore be raised by 84t/hr. Consequently, the total live steam consumption in the plantincreases by 28 t/hr resulting in a consumption of 119.9 t/hr instead of91.9 t/hr.

Thus, in a Bayer process bauxite digestion plant, an increase of 10 g/lin the active Na₂O concentration of the attack liquor, brings about anincrease of 28 t/hr in the live steam consumption, which renders thesaid increase in concentration of the spent liquor (or liquor intendedfor use in digestion) economical undesirable.

According to the invention, the previously mentioned industrial unit wasequipped with the double stage spent liquor simultaneous heating andevaporation section (FFES). The comparison of figures between theprevious art (table 2, column 4) and the subject of this invention(table 2, column 5) shows that, by implementing the simultaneousheater-evaporator (FFES), the spent liquor temperature being raised from120° C. (H₁ outlet) to 156° C. (H₀ outlet), it was possible to reach theset objective, that is to say, an increase in the concentration ofactive Na₂O from 130 to 140 g/l in the spent liquor without asignificant rise in the live steam consumption. To achieve this, thetotal evaporation capacity of the flashing zone E (line 7) was raisedfrom 86.6 t/hr to 170.3 t/hr, which represents an increase in totalevaporation capacity of 84 t/hr as compared with the previous art(columns 1 and 2) for an increased steam consumption of only 1 t/hr(92.9 t/hr against 91.9 t/hr) (line 13, columns 1, 2, 4 and 5).

Thus, by means of the invention, with an additional live steamconsumption of 1 t/hr as compared with the status of the technique, theevaporation capacity of the plant is increased by 84 t/hr. Consequently,the set objective of increasing the active Na₂O concentration in thespent liquor intended for attack is reached with a very slight rise inthe live steam consumption (1 t/hr), whereas the same plant, notequipped with the invention, could only reach this objective with anadditional live steam consumption of 28 t/hr.

The integration in the heating section of an alumina production unit(according to the previous art) of a simultaneous heating andevaporation section according to the invention, allows the simultaneousadjustment of active Na₂O in the spent liquor intended for use indigestion, and the liquor temperature to the right level for aluminaproduction and, consequently, considerably improves the overall energyefficiency of the Bayer process with a low investment cost, which can bepaid off in a short time.

TABLE 2 BAYER PROCESS Plant operating according to previous art presentstatus (1) present status (2) Solution to achieve set objective withoutspecific with specific Objective set for according to the evaporationevaporation process operation according to invention with Line section Psection P (3) previous art 2 stages  1 Annual production of Al₂O₃ (t/hr)500000 500000 500000 500000 500000  2 Spent liquor flow - outlet H₁(t/hr) 1460 1460 1460 1460 1460  3 Concentration of active Na₂O (in g/l)in spent 130 130 130 130 130 liquor - inlet H₁  4 Liquor temperature -outlet H₁ (° C.) 120 120 120 120 120  5 Liquor temperature - ouletheating section (H₁ or 156 156 156 156 156 FFES) (° C.)  6 Na₂Oconcentration in spent liquor (in g/l) - outlet 130 130 140 140 140 ofheating section  7 Evaporation capacity (in t/hr) of the staged 86.386.3 — 86.3 170.3 flashing zone (E)(*)  8 Evaporation capacity (in t/hr)of the evaporation 0 0(**) — 84 (in P) 0 zone (P) heated with live steam 9 Total evaporation capacity of the plant (E) + (P) 86.3 86.3 170.3170.3 170.3 (in t/hr)(*) 10 Increase in total evaporation capacity ofthe plant 0 0 84 — 84 (in t/hr) 11 Live steam consumption in zone (E)(in t/hr) 91.9 91.9 — 91.9 92.9 12 Live steam consumption in zone (P)(in t/hr) 0 0 — 28 0 13 Total live steam consumption in (E) + (P) (int/hr) 91.9 91.9 — 119.9 92.9 (*)The evaporation capacity of zone E iscalculated to account for the flashing of the pregnant liquor from 145°C. to 107.3° C. (atmospheric pressure) (**)The hypothesis is made tosimplify the explanation that the evaporation section (P) has a capacityof zero before setting the objective of increasing the active Na₂Oconcentration in the spent liquor by 10 g/l

Comparative Example No. 3 Complying with FIGS. 4 and 15

According to the status of the technique, a conventional aluminaproduction plant not equipped with a specific evaporation section (P)(according to FIGS. 1 and 3), with a capacity of 500,000 t/yearaccording to the previous art (table 3, column 1) includes:

a flash-evaporation section in stages FT, interconnected, installed inseries in which circulates the aqueous medium coming from digestion;

a section of heaters (H), interconnected, installed in series and inwhich circulates the spent liquor (aqueous medium intended for use indigestion) counter-current to the aqueous medium coming from digestion,each flash tank (FT) being connected for vapour circulation with adirectly corresponding heater (H);

a heater H₀ connected to a source of live steam (LS), and also to theinlet of mixer (M) [to ensure the circulation of the aqueous mediumintended for use in digestion coming from the outlet of heater H₁ of theseries of heaters (H)]

The spent liquor (coming from the precipitation of Al(OH)₃) has:

a flowrate of about 1460 t/hr at the outlet of H,

a concentration of about 130 g/l active Na₂O at the inlet of H₁,

a temperature of about 120° C. at the outlet of H₁,

a temperature of about 156° C. at the outlet of H₀,

finally, a concentration of about 130 g/l active Na₂O at the outlet ofH₀.

To raise the temperature of the spent liquor from 120° C. (at the outletof H₁) to 156° C. (at the outlet of H₀), so that the attack liquor (madeup of the mixture of the said liquor with the heated bauxite) reachesthe temperature level required for bauxite digestion (about 145° C.),the evaporation capacity of the staged flashing zone is 86.3 t/hr and itconsumes 91.9 t/hr of live steam for the operation of the plant.

In the same conventional alumina production unit, but equipped with aspecific evaporation section (P) (according to FIGS. 2 and 3), having acapacity of 500,000 t/year complying with the previous art (table 3,column 2), and the previously mentioned conditions, the temperature riseof the spent liquor from 120° C. (at the outlet of H₁) to 156° C. (atthe outlet of H₀), for the attack medium, made up of the mixture of thesaid liquor and the heated bauxite, to be at the required temperaturefor the digestion of this bauxite (about 145° C.), the total evaporationcapacity of the plant is still 86.3 t/hr and it consumes 91.9 t/hr oflive steam for the operation of the said plant.

The Bayer process industrial plant is therefore set to operate at aconcentration of 145 g/l active Na₂O in the spent liquor instead of 130g/l, at the outlet of the heating section (table 3, column 3, line 6).

In this same plant according to the previous art, to increase the activeNa₂O concentration in the spent liquor from 130 g/l to 145 g/l (table 3,column 2) while maintaining the temperature of the liquor at the outletof H₀ at 156° C., the total evaporation capacity of the plant must beincreased from 86.3 t/hr to 211.3 t/hr (line 9), which requires anincrease of 125 t/hr total evaporation capacity in the plant (column 3,line 10).

To achieve the set objective, that is to say to accelerate bauxitedigestion and increase Al₂O₃ productivity[in the plant according to thestatus of the technique, equipped with a specific evaporation section(P)] (table 3, column 4), the total evaporation capacity required in theplant must be increased from 86.3 t/hr to 211.3 t/hr. The evaporationcapacity of the specific evaporation section (P) must therefore beraised by 125 t/hr. Consequently, the total live steam consumption inthe plant increases by 42 t/hr resulting in a consumption of 133.9 t/hrinstead of 91.9 t/hr.

Thus, in a Bayer process bauxite digestion plant according to the statusof the technique, an increase of 15 g/l in the active Na₂O concentrationof the attack liquor, brings about an increase of 42 t/hr in the livesteam consumption, which renders the said increase in concentration ofthe spent liquor (or liquor intended for use in digestion) economicalundesirable.

According to the invention, the previously mentioned industrial unit wasequipped with the triple stage spent liquor simultaneous heating andevaporation section (FFES). The comparison of figures between theprevious art (table 3, column 4) and the subject of this invention(table 3, column 5) shows that, by implementing the simultaneousheater-evaporator (FFES) according to the invention, the spent liquortemperature being raised from 120° C. (H₁ outlet) to 156° C. (H₀outlet), it was possible to reach the set objective, that is to say, anincrease in the concentration of active Na₂O from 130 to 145 g/l in thespent liquor. To achieve this, the total evaporation capacity of theflashing zone E (line 7) was raised from 86.6 t/hr to 211.3 t/hr, whichrepresents an increase in total evaporation capacity of 125 t/hr ascompared with the previous art (columns 1 and 2) for an increased steamconsumption of 11.4 t/hr. So with almost four time less live steam, itwas possible to generate 125 t/hr evaporation while providing about 10t/hr of steam at 2.4 bar abs., capable of producing savings in livesteam in the specific evaporation section (P) representing 20 to 40% ofthe total live steam initially consumed, depending on the exactconfiguration of the evaporation section. In this case, the plantequipped in this way provides 10 t/hr of steam (line 10) at 2.4 bar abs.for other applications.

The integration in the heating section of an alumina production unit(according to the previous art) of a simultaneous heating andevaporation section according to the invention, allows the simultaneousadjustment of active Na₂O in the spent liquor intended for use indigestion, and the liquor temperature to the right level for aluminaproduction and, consequently, considerably improves the overall energyefficiency of the Bayer process with a low investment cost, which can bepaid off in a short time.

TABLE 3 BAYER PROCESS Plant operating according to previous art presentstatus (1) present status (2) Solution to achieve set objective withoutspecific with specific Objective set for according to the evaporationevaporation process operation according to invention with Line section Psection P (3) previous art 2 stages  1 Annual production of Al₂O₃ (t/hr)500000 500000 500000 500000 500000  2 Spent liquor flow - outlet H₁(t/hr) 1460 1460 1460 1460 1460  3 Concentration of active Na₂O (in g/l)in spent 130 130 130 130 130 liquor - inlet H₁  4 Liquor temperature -outlet H₁ (° C.) 120 120 120 120 120  5 Liquor temperature - ouletheating section (H₁ or 156 156 156 156 156 FFES) (° C.)  6 Na₂Oconcentration in spent liquor (in g/l) - outlet 130 130 145 145 145 ofheating section  7 Evaporation capacity (in t/hr) of the staged 86.386.3 — 86.3 211.3 flashing zone (E)(*)  8 Evaporation capacity (in t/hr)of the evaporation 0 0(***) — 125 (in P) 0 zone (P) heated with livesteam  9 Total evaporation capacity of the plant (E) + (P) 86.3 86.3211.3 211.3 211.3 (in t/hr)(*) 10 Increase in total evaporation capacityof the plant 0 0 125 — 125 (in t/hr) 11 Live steam consumption in zone(E) (in t/hr) 91.9 91.9 — 91.9 103.3 10(**) 12 Live steam consumption inzone (P) (in t/hr) 0 0 — 42 0 13 Total live steam consumption in (E) +(P) (in t/hr) 91.9 91.9 — 133.9 103.3 (*)The evaporation capacity ofzone E is calculated to account for the flashing of the pregnant liquorfrom 145° C. to 107.3° C. (atmospheric pressure) (**)Steam flow at 2.4bar abs, useful for other applications in the plant (***)The hypothesisis made to simplify the explanation that the evaporation section (P) hasa capacity of zero before setting the objective of increasing the activeNa₂O concentration in the spent liquor by 15 g/l

What is claimed is:
 1. A process line for use in a Bayer processinstallation for the treatment of bauxite wherein a slurry of bauxiteore in an alkaline aqueous medium is digested at temperature andpressure values to dissolve alumina as sodium aluminate and separatedinto solids and a pregnant liquor stream from which aluminum hydroxideis precipitated leaving aqueous medium, the process line to raisesimultaneously the temperature and concentration of the aqueous mediumfor use in the bauxite ore digestion, including: a section ofinterconnected evaporator-flash tanks FT₁ to FT_(n) installed in seriesand through which circulates an aqueous medium coming from the bauxiteore digestion in a digester D; a section of interconnected heaters H₀,H₁ to H_(n) installed in series through which circulates the aqueousmedium for use in the bauxite ore digestion, each evaporator FT₁ toFT_(n) flash tank being connected with each heater H₁ to H_(n) directlyassociated with each flash tank FT₁ to FT_(n) between the flash tanksand the heaters for obtaining vapor circulation; an optional specificevaporation section P for eliminating by evaporation a quantity ofwater, additional to that removed in the evaporator-flash tank FT₁ toFT_(n); at least one heater H₀ connected to a source of live steaminstalled at the inlet of a mixing device and/or bauxite ore digester bymeans of the aqueous medium to raise the temperature of the aqueousmedium coming from the outlet of the first heater H₁ in the heatersection H₀, H₁ to H_(n) to the temperature required for digestion; and asimultaneous heating and evaporation section for the aqueous medium usedin the digestion.
 2. A process line according to claim 1 wherein saidsimultaneous heating and evaporation section is inserted at any point inthe heating section of the heaters H₀, H₁ to H_(n).
 3. A process lineaccording to claim 1 wherein said simultaneous heating and evaporationsection is inserted at any point in the heating section of the heatersH₀, H₁ to H_(n) of the specific evaporation section P.
 4. A process lineaccording to claim 1 wherein said simultaneous heating and evaporationsection is inserted simultaneously at any point in the heating sectionof the heaters H₀, H₁ to H_(n) and of the specific evaporation sectionP.
 5. A process line according to claim 1 wherein the simultaneousheating and evaporation section is inserted between heater H₀ fed withlive steam, in the heating section of the heaters H₀, H₁ to H_(n), andfrom which said simultaneous heating and evaporation section receivesthe aqueous medium to be treated prior to its use in bauxite oredigestion, and the digestion zone which it feeds with aqueous medium forbauxite ore digestion at the temperature and concentration required fordigestion.
 6. A process line according to claim 1 wherein thesimultaneous heating and evaporation section is inserted between heaterH₀ fed with live steam in the specific evaporation section P, and fromwhich said simultaneous heating and evaporation section receives theaqueous medium to be treated prior to its use in bauxite ore digestion,and heater H_(n) of the section of interconnected heaters H₀, H₁ toH_(n) fed with flash vapor from the evaporator flash tanks, which itfeeds with treated aqueous medium.
 7. A process line according to claim1 wherein the simultaneous heating and evaporation section is insertedbetween heater H₁ fed with flash vapor in the heating section of theheaters H₀, H₁ to H_(n) and/or in the heating section of the heaters H₀,H₁ to the specific evaporation section P and from which saidsimultaneous heating and evaporation section receives the aqueous mediumto be treated prior to its use in bauxite ore digestion, and heater H₀fed with live steam in the heating section of the H₀, H₁ to H_(n) and/orin the heating section of the heaters H₀, H₁ to H_(n) of the specificevaporation section P, which it feeds with treated aqueous medium.
 8. Aprocess line according to claim 1 wherein the simultaneous heating andevaporation section is inserted: according to a first level, betweenheater H₁ fed with flash vapor FT₁ and from where it receives theaqueous medium to be treated, for use in bauxite ore digestion, andheater H₀ in the heating section of the heaters H₀, H₁ to H_(n) and/orin the heating section of the heaters H₀, H₁ of the specific evaporationsection P fed with live steam, in the heating section of the heaters H₀,H₁ to H_(n) and/or in the heating section of the heaters H₀, H₁ of thespecific evaporation section P; according to a second level, betweenheater H₀ fed with live steam and which receives the aqueous medium at ahigher temperature for use in bauxite ore digestion and; either thebauxite ore digestion zone which it feeds with aqueous medium heatedand/or concentrated to the level for bauxite ore digestion; or, in thecase of the specific evaporation section P, the heater H_(n) to which itfeeds the treated aqueous medium.
 9. A process line according to claim 1wherein the simultaneous heating and evaporation section is fed withlive steam and produces vapor which is used in situ.
 10. A process lineaccording to claim 1 wherein the simultaneous heating and evaporationsection is connected for the use of the vapor produced: to heater H₀,combining its live steam feed with all or part of the vapor produced;and/or to at least one of the heaters H₁ to H_(n) in the heating sectionand/or the specific evaporation section P; and/or to one of the effectsof the multiple effect of the specific evaporation section P; and/or tothe means for the production of hot water and/or to heat the spentliquor for use in preparing bauxite ore slurry for bauxite oredigestion.
 11. A process line according to claim 1 wherein thesimultaneous heating and evaporation section includes at least onesimultaneous heating and evaporation stage for the aqueous medium, eachstage being composed of one said simultaneous heating and evaporationsection or several simultaneous heating and evaporation means installedin series or in parallel.
 12. A process line according to claim 11wherein the simultaneous heating and evaporation section includes atleast one single stage fed with live steam and connected for treatingthe vapor produced, to heater H₀ combining the live steam feed of saidone single stage with all or part of the vapor and/or to the exchangerof said one single stage.
 13. A process line according to claim 11wherein the simultaneous heating and evaporation section includes twosimultaneous heating and evaporation stages in which the first stage isconnected, to treat the vapor produced, to the second stage and toheater H₀.
 14. A process line according to claim 13 wherein one ofstages in the simultaneous heating and evaporation section is connectedupstream to the outlet of heater H₁ in the series of heaters H anddownstream to the inlet of heater H₀ and in that the other stage of thesimultaneous heating and evaporation section is connected upstream tothe outlet of heater H₀ and downstream to the digester D when thesimultaneous heating and evaporation section is installed in the sectionof heaters or from heater H_(n) when the simultaneous heating andevaporation section is installed in the specific evaporation section P.15. A process line according to claim 11 wherein the simultaneousheating and evaporation section includes three simultaneous heating andevaporation stages, in which one of the stages is connected to theoutlet of heater H₀, fed with live steam in the heating and/orevaporating sections, the other stages in series being connectedupstream to the outlet heater H₁ fed with flash vapor, and downstream tothe inlet of heater H₀ of the heating and/or evaporating sections.
 16. Aprocess line according to claim 11 wherein one stage of the simultaneousheating and evaporation section includes a falling film simultaneousheater-evaporator which in turn includes a vertical tube exchanger, adistribution system to distribute the spent liquor on the vertical tubeexchanger and a liquid-vapor separator.
 17. A process line according toclaim 11 wherein one stage of simultaneous heating and evaporationsection also includes a direct contact heater to heat the aqueous mediumintended for use in digestion with vapor.
 18. In a Bayer processtreatment of bauxite where a slurry of bauxite in an alkaline aqueousmedium is digested at high temperature and pressure values required todissolve alumina as sodium aluminate and separated into solids and apregnant liquor stream from which aluminum hydroxide is precipitatedleaving aqueous medium, the improvement for raising simultaneously thetemperature and concentration of said alkaline aqueous medium from whichaluminum hydroxide has been separated for its use in the furtherdigestion of bauxite, comprising the steps of a) cooling and evaporatingthe aqueous medium in a section of interconnected flash evaporators FT₁to FT_(n) installed in series; b) separating a the sterile impuritiesresidue from the pregnant liquor, precipitating the aluminum hydroxidefrom the sodium aluminate present in the aqueous medium, separating outthe aluminum hydroxide and recovering the remaining aqueous medium foruse in the further digestion of bauxite; c) evaporating a quantity ofwater in addition to that removed in the flash evaporators FT₁ to FT_(n)in an evaporation section P inserted between the section for theprecipitation and separation of aluminum hydroxide from the pregnantliquor and recovery of spent liquor in order to further concentrate theaqueous medium for further digestion of bauxite; d) heating the aqueousmedium in a section of interconnected heaters H₀, H₁ to H_(n) installedin series with each flash evaporator FT₁ to FT_(n) directly associatedwith each heater H₀, H₁ to H_(n); e) using the flash vapor generated ineach flash evaporator FT₁ to FT_(n) to concentrate the aqueous mediumrecovered in step b) and forming the aqueous medium to be used in thedigestion of bauxite, in the heaters H₀, H₁ to H_(n); f) utilizing livesteam supplied to a heater H₀ to raise the temperature of the aqueousmedium coming from the heaters to the temperature required fordigestion; and g) carrying out a simultaneous heating and evaporation ofthe aqueous medium for use in the digestion of bauxite in a simultaneousheating and evaporation section.
 19. Process according to claim 18wherein the simultaneous heating and evaporation step g) of the aqueousmedium is inserted at any one point in step d).
 20. Process according toclaim 18 wherein said simultaneous heating and evaporation step g) isinserted at any point in evaporation section P of step c) when step c)is provided with the same number of flash evaporators FT₁ to FT_(n) asprovided in evaporation section a) and the same number of heaters H₀, H₁to H_(n) as in heating step d).
 21. Process according to claim 18wherein said simultaneous heating and evaporation step g) is insertedsimultaneously at any one point in steps c) and d) when step c) isprovided with the same number of flash evaporators FT₁ to FT_(n) asprovided in evaporation step a) and the same number of heaters H₀, H₁ toH_(n) as in heating step d).
 22. Process according to claim 18 whereinthe aqueous medium is fed to the simultaneous heating and evaporationstep g) to be heated and/or evaporated from the heating step d) after ithas been fed with live steam in heater H₀ of step d) and provides theaqueous medium for bauxite digestion at the temperature andconcentration required for said digestion.
 23. Process according toclaim 18 wherein the simultaneous heating and evaporation step g) of theaqueous medium is conducted with the aqueous medium from heater H₀ afterit has been fed with live steam and the heated and/or concentratedaqueous medium is then fed to a successively arranged heater in heatingstep d).
 24. Process according to claim 18 wherein the simultaneousheating and evaporation step g) of the aqueous medium from the firststage of the heating is fed with flash vapor from step a) and saidaqueous medium is then delivered to the heating step after having beenfed with live steam, where it is heated and/or concentrated.
 25. Processaccording to claim 18 wherein the simultaneous heating and evaporationstep g) of the aqueous medium to be heated and/or evaporated from thefirst stage of the heating is fed with flash vapor from step a), saidaqueous medium is then delivered to the heating stage and fed with livesteam whereby the aqueous medium is raised to a higher temperature, saidaqueous medium is recovered from the heating stage, fed with live steamand is further heated and/or concentrated and the heated and/orconcentrated aqueous medium is then fed to the digestion for digestionof further bauxite ore.
 26. Process according to claim 18 wherein ininstances where the evaporation section P of step c) includes a multipleeffect, the simultaneous heating and evaporation section of step g) isinserted in step d).
 27. Process according to claim 18 wherein thesimultaneous heating and evaporation step g) of the aqueous medium isfed with live steam and the vapor that the simultaneous heating andevaporation produces is utilized in step g).
 28. Process according toclaim 18 wherein the vapor produced in the simultaneous heating andevaporation step g) is fed to at least one of the heating stages H₀, H₁to H_(n) of step d) when step a) is equipped with the same number offlash evaporators FT₁ to FT_(n) in step a) as heaters in heating stepd).
 29. Process according to claim 26 wherein the vapor produced in thesimultaneous heating and evaporation step g) is in addition consumed inthe specific evaporation section P of step c).
 30. Process according toclaim 18 wherein the vapor produced in the simultaneous heating andevaporation step g) is in addition consumed in at least one additionalheater inserted for this purpose between at least two of the heaters H₀,H₁ to H_(n) in step d).
 31. Process according to claim 18 wherein thevapor produced in the simultaneous heating and evaporation step g) is inaddition consumed to heat the spent liquor of step b) used in thepreparation of bauxite ore slurry prior to the bauxite ore digestionand/or to produce water having a temperature of between 80 to 90° C. 32.Process according to claim 18 wherein the simultaneous heating andevaporation step g) of the aqueous medium is carried out in at least onestage, each stage comprising one or several simultaneous heating andevaporation means operating in series or in parallel.
 33. Processaccording to claim 32 wherein the simultaneous heating and evaporationstep g) of the aqueous medium is carried out in at least two stages andwherein the vapor generated by one of the simultaneous heating andevaporating stages is used to feed the simultaneous heating andevaporation stage directly preceding it.
 34. Process according to claim32 wherein live steam is fed to the first simultaneous heating andevaporation stage of said simultaneous heating and evaporation step g)for heating the aqueous medium.
 35. Process according to claim 32wherein one of the simultaneous heating and evaporation sections forheating and evaporating the aqueous medium of step g) is locatedfollowing the final heater of step d) and another of said simultaneousheating and evaporation sections for heating and evaporating aqueousmedium is inserted between the outlet of heater H₁ of step d) and theintroduction of live steam into the heater H₀ of step d).
 36. Processaccording to claim 32 wherein one of the simultaneous heating andevaporation sections for heating and evaporating the aqueous medium ofstep g) is located at the outlet of heater H₀ and two other simultaneousheating and evaporation sections are inserted between the outlet ofheater H₁ of step d) and the inlet of live steam into the heater H₀ instep f).
 37. Process according to claim 18 wherein in the simultaneousheating and evaporating step g), a vertical falling film of aqueousmedium is used, heated by vapor, and wherein the said film generatesvapor in situ which is thereafter separated from the aqueous mediumresulting from the simultaneous heating and evaporation.
 38. Processaccording to claim 18 wherein aqueous medium is heated by direct contactof said aqueous medium with heated vapor injection.
 39. Processaccording to claim 38 wherein the aqueous medium intended for use inbauxite ore digestion is heated in at least one stage by heated vaporinjection, and wherein the stage is located at the inlet of thesimultaneous heating and evaporation step g).
 40. Process according toclaim 38 wherein in each of the direct contact heating stages, vaporgenerated by each of the simultaneous heating and evaporation stages forthe aqueous medium is utilized.
 41. Process according to claim 18wherein the vapor produced in at least one of the simultaneous heatingand evaporation stages of the simultaneous heating and evaporation stepg) is compressed to raise its pressure to that of the operating pressurein the said stage of the simultaneous heating and evaporation of stepg).